Such a trolley, which comprises a mobile casing, is particularly useful in surgical operating theatres to provide, at any desired location, a sterile surface on which surgical instruments and associated items can be unwrapped and laid. Additionally, by suitable positioning, such a trolley can function to minimise, indeed substantially prevent, airborne contamination of the operating site (wound) as well as gloved hands, gowns, drapes etc. which are introduced into the area of the operating site. Of course, other applications of such a trolley are possible, for example in providing a local sterile area for purposes of pharmaceutical or electronic production steps where a full clean room environment is not available or needs to be supplemented. A sterile air trolley has been previously proposed in UK patent specification No. 2108846. In that proposal an upper region of the trolley extended laterally beyond the lower region and outlets for sterile air were distributed throughout that upper region, including the upper surface thereof, which provided the sterile surface for placement of instruments.